Modern industrial machines are capable of achieving extremely accurate and repeatable motion. Whether it is a high-speed bottling plant or a complex robotic arm, every movement happens with precision and consistency. This level of accuracy is not accidental. It is achieved using a technology known as closed loop motion control with servo systems. In this blog, we explain servo motion control with PLC, how closed loop control works, the role of PLC, servo drive, servo motor, and encoder, along with a practical overview of configuring a servo axis in Siemens S7-200 Smart PLC. This guide is written for automation engineers, PLC programmers, maintenance professionals, and beginners in industrial automation.
This guide is ideal for automation engineers, PLC programmers, maintenance professionals, and beginners in industrial automation.
What Is Closed Loop Control in Servo Systems?
Closed loop control is a control technique where the system continuously checks whether the actual position of the motor matches the commanded position given by the PLC. Unlike open loop systems, the servo system does not simply execute commands blindly.
In a closed loop system, the motor position is constantly monitored. If there is any deviation between the desired position and the actual position, the system immediately corrects it. This continuous correction process ensures high accuracy, smooth motion, and reliable operation even under load variations.
The most important concept in closed loop control is feedback, which allows the system to self-correct in real time.
Servo Motion Control as a Team
A servo motion control system works like a coordinated team where each component has a defined responsibility. This teamwork is what makes precise motion possible.
1. PLC (Controller – The Brain)
The PLC acts as the brain of the system. It decides how far the motor should move, how fast it should rotate, and in which direction the movement should happen. All motion-related decisions originate from the PLC program.
2. Servo Drive (Translator & Amplifier)
The servo drive acts as a translator and power amplifier. PLC outputs are low-power signals and cannot drive a motor directly. The servo drive converts these low-level signals into high-power electrical output by controlling voltage and current. Without the servo drive, the PLC would not be able to control the motor.
3. Servo Motor (Muscle)
The servo motor acts as the muscle of the system. It performs the actual mechanical movement with high torque, accurate positioning, and smooth speed control.
Role of Encoder in Closed Loop Control
The encoder is one of the most critical components in a servo system. It is mounted on the servo motor shaft and continuously measures the motor’s actual position.
This real-time position data is sent back to the servo drive and PLC as feedback. Using this feedback, the system compares the actual position with the target position and immediately corrects any error. Without an encoder, true closed loop control is not possible, and motion accuracy would be compromised.
How PLC and Servo Communicate: Pulse Train Output (PTO)
In many industrial servo applications, the PLC communicates with the servo drive using Pulse Train Output (PTO). This communication method is simple yet extremely precise.
One pulse signal defines the speed and distance of movement, while another signal controls the direction of rotation. By generating accurate pulses, the PLC can precisely control how far and how fast the servo motor moves.
Important Servo Motion Control Terms
Understanding a few essential motion control terms makes servo programming much easier.
Electronic gear ratio
Defines the relationship between PLC pulses and real-world movement. For example, a configuration may define that 1000 pulses correspond to 10 mm of linear travel. This mapping allows accurate conversion between digital commands and physical motion.
Homing
Is used to establish a reference zero position when the machine starts. It ensures that every motion sequence begins from a known and repeatable point.
Absolute move
Commands the servo motor to move to a fixed position, such as moving directly to position 100 mm regardless of the current position.
Relative move
Commands the motor to move a specified distance from its current position, such as moving 50 mm forward from wherever it is currently located.
Hardware Requirements for Servo Control with PLC
1. PLC Output Type
Correct hardware selection is essential for stable servo operation. When using a PLC for servo motion control, the PLC must have transistor-type outputs. Relay outputs are not suitable because they cannot generate high-speed pulse signals.
In the case of Siemens S7-200 Smart PLC, dedicated high-speed outputs such as Q0 and Q1 are used for pulse and direction signals.
2. Voltage Protection Using Resistor
Voltage protection is another critical consideration. PLC outputs typically operate at 24V, while servo drive pulse inputs usually work at 5V. To prevent damage to the servo drive, a 2K ohm resistor must be used. This simple component protects the drive from overvoltage and ensures long-term reliability.
Software Configuration Using Motion Wizard
Siemens provides a built-in Motion Wizard that simplifies servo configuration significantly. Instead of manually calculating every parameter, the wizard guides the user through a structured setup process.
The configuration starts by selecting the motion unit, such as pulses, millimeters, or inches. Acceleration and deceleration values are then set to ensure smooth motor movement without mechanical stress. Pulse and direction outputs are mapped to the actual PLC hardware outputs, and finally, all settings are saved.
The wizard automatically calculates internal values based on the electronic gear ratio, although these values can be fine-tuned if required.
Servo Axis Configuration in S7-200 Smart PLC
Pulse Output Selection
Common options include:
- Single-phase two-output
- Two-phase output
- Quadrature pulse
In this setup:
- One pulse output controls speed and distance
- Second output controls direction
Motor Speed Settings
You can configure:
- Maximum motor speed
- Start/stop behavior
- Motion response characteristics
Jog Speed
Jog speed defines how fast the motor moves during manual jogging or inching operations.
Acceleration and Deceleration Time
Proper tuning:
- Reduces mechanical shock
- Prevents jerks
- Improves system life
Reading Actual Motor Position
By enabling position reading, the PLC can continuously monitor the actual servo motor position. This feedback is useful for diagnostics, motion verification, and safety logic. It allows the system to confirm that commanded movements are being executed correctly.
Programming Servo Motion Instructions
1. Load Position Instruction
The Load Position instruction is used to initialize or reset the servo position. It is commonly executed before starting motion to clear any previous position data and establish a known reference.
2. Go To Position Instruction
The Go To Position instruction commands the servo to move to a defined position at a specified speed. It supports both absolute and relative motion modes and includes an abort option to stop the motor immediately during an emergency.
3. Axis Control Instruction
The Axis Control instruction enables and initializes the servo axis. It is typically kept permanently enabled during machine operation.
4. Manual Jog Instruction
The Manual Jog instruction allows controlled clockwise or anticlockwise movement. This mode is mainly used during testing, setup, and manual positioning tasks.
Practical Wiring and Safety Tips
Use Shielded Cables
Pulse and direction signals are sensitive to electrical noise. Using shielded cables helps prevent interference from nearby machines and ensures reliable communication.
Common Ground Connection
The PLC 0V and servo drive 0V must be connected to a common ground. Without a common reference, signal instability and positioning errors may occur.
Emergency Stop Is Mandatory
A hardware emergency stop (E-Stop) is mandatory in all servo systems. Software-based emergency stops should never be trusted alone. A physical emergency stop button is a strict safety requirement and has no substitute.
Conclusion: Servo Motion Control with PLC
Servo motion control using closed loop control enables precise, smooth, and reliable machine movement. By combining PLC logic, servo drives, servo motors, and encoder feedback, industries achieve high accuracy even in demanding applications.
With correct hardware selection, proper wiring, and structured programming, Siemens S7-200 Smart PLC servo configuration becomes straightforward and efficient.
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